Conventionally, wiring is formed in minute wiring grooves, holes, or resist openings provided on surfaces of substrates such as semiconductor wafers or bumps (bumpy electrodes) used to electrically connect to electrodes of a package which are formed on the surfaces of the substrates. As a method for forming the wiring and bumps, an electrolytic plating process, vacuum deposition process, printing process, ball bumping process, and the like are known, for example. With increases in I/O counts and pitch refinement on semiconductor chips, the electrolytic plating process which allows refinement and shows comparatively stable performance has come to be used frequently.
When wiring or bumps are formed by the electrolytic plating process, a seed layer (feeder layer) with low electrical resistance is formed on surfaces of barrier metal provided in the wiring grooves, holes, or resist openings in the substrates. A plating film grows on a surface of the seed layer. In recent years, seed layers with thinner film thickness have come to be used along with refinement of wiring and bumps. With decreases in the film thickness of the seed layer, the electrical resistance (sheet resistance) of the seed layer increases.
Generally, a substrate to be plated has an electrical contact on its periphery. Consequently, an electric current corresponding to combined resistance of an electrical resistance value of the plating solution and electrical resistance value of the seed layer between a central portion of the substrate and the electrical contact flows through the central portion of the substrate. On the other hand, an electric current almost corresponding to the electrical resistance value of the plating solution flows through the periphery (near the electrical contact) of the substrate. That is, the flow of the electric current to the central portion of the substrate is resisted to an extent corresponding to the electrical resistance value of the seed layer between the central portion of the substrate and the electrical contact. The phenomenon in which electric current concentrates on the periphery of a substrate is referred to as a terminal effect.
In the case of a substrate which has a seed layer comparatively thin in film thickness, the electrical resistance value of the seed layer between the central portion of the substrate and the electrical contact is comparatively high. Therefore, in plating a substrate whose seed layer is comparatively thin in film thickness, the terminal effect is prominent. Consequently, the plating rate in the central portion of the substrate falls, making the plating film in the central portion of the substrate thinner in film thickness than the plating film in the periphery of the substrate and resulting in reduced in-plane uniformity of film thickness.
In order to curb the reduction in the in-plane uniformity of film thickness due to the terminal effect, it is necessary to adjust an electric field applied to the substrate. For example, a plating apparatus is known, in which an anode regulation plate is installed on a front face of an anode to regulate a potential distribution on an anode surface (see Japanese Patent Laid-Open No. 2005-029863).
Now, the influence of the terminal effect varies with the degree of film thickness of the seed layer on the substrate. Specifically, as described above, when the seed layer is comparatively thin in film thickness, since the sheet resistance is comparatively high, the influence of the terminal effect appears prominently. On the other hand, when the seed layer is comparatively thick in film thickness, since the sheet resistance is comparatively low, the influence of the terminal effect is comparatively small.
Also, the influence of the terminal effect can vary not only with the degree of film thickness of the seed layer, but also with the other factors. For example, when a resist aperture ratio of the substrate is comparatively high, the plating film formed on the substrate has a comparatively large area, where the resist aperture ratio is the area ratio of a portion not covered with resist (open portion of the resist) to a region bordered by an outer edge of the resist. Therefore, as the plating film is formed on the substrate, the formed plating film causes electric current to flow readily in the central portion of the substrate as well. In other words, as the plating film is formed on the substrate, the electrical resistance value between the central portion of the substrate and the electrical contact decreases, gradually reducing the influence of the terminal effect. On the other hand, when the resist aperture ratio of the substrate is comparatively low, the area of the plating film formed on the substrate is relatively small. Consequently, when the resist aperture ratio of the substrate is comparatively low, even if a plating film is formed on the substrate, variation in the electrical resistance value between the central portion of the substrate and the electrical contact is smaller than when the resist aperture ratio of the substrate is comparatively high, and thus the influence of the terminal effect remains large.
Also, when the electrical resistance value of a plating solution used to process the substrate is comparatively high, the influence of the terminal effect is smaller than when the electrical resistance value of the plating solution used to process the substrate is comparatively low. Specifically, if the electrical resistance value of the plating solution is R1 and the electrical resistance value of the seed layer between the central portion of the substrate and the electrical contact is R2, an electric current corresponding to combined resistance value (R1+R2) flows through the central portion of the substrate. On the other hand, an electric current almost corresponding to the electrical resistance value R1 of the plating solution flows through the periphery (near the electrical contact) of the substrate. Thus, as the electrical resistance value R1 increases, the influence of the electrical resistance value R2 to the electric current flowing through the central portion of the substrate decreases, reducing the influence of the terminal effect.
In this way, the influence of the terminal effect varies with characteristics of the substrate, conditions for processing the substrate, and the like. Therefore, when plural substrates differing in the influence of the terminal effect are plated using a single plating apparatus, in order to curb the reduction in the in-plane uniformity of film thickness due to the terminal effect, it is necessary to adjust the electric fields applied to the substrates, according to the characteristics of the respective substrates, conditions for processing the substrates, and the like. However, in order to adjust the electric fields according to the characteristics of the substrates, conditions for processing the substrates, and the like using an anode regulation plate such as described in Japanese Patent Laid-Open No. 2005-029863, it is necessary to prepare plural anode regulation plates which suit the characteristics of the substrates, conditions for processing the substrates, and the like.
Besides, even if plural anode regulation plates are prepared, each time substrates differing in characteristics and processing conditions are processed, it is necessary to take the anode regulation plate out of the plating bath and install another anode regulation plate, involving time and effort.